2012-02-04

Hour-by-hour Comparison of Beijing's PM2.5 Pollution

Public outcry over the air quality in Beijing has impelled the government to start releasing PM2.5 pollution data, something the U.S. Embassy has been doing via Twitter since 2009. For some background on "PM2.5", see this post on the air pollution situation in Beijing. In short, PM2.5 is a dangerous form of pollution, measured in micrograms per cubic meter. The WHO's standard for a maximum acceptable yearly average is 10 μ/m3; Beijing regularly sees days over 200 or 300 μ/m3.

Now that we have an official source of hourly PM2.5 measurements in Beijing, it seems appropriate to do an hour-by-hour comparison with the embassy's, duly noting that different values from each station do not necessarily indicate inaccurate data from either source. The U.S. Embassy takes their measurements in eastern Beijing near Liangmaqiao and the Beijing Municipal Environmental Monitoring Center (BJMEMC) takes theirs in the west near Chegongzhuang, a six-mile distance. The different environments and equipment undoubtedly add to variability. In the past, local officials have stated that the embassy's location near construction sites and a major road inflate their measurements. (As a side note, if that is the case, the embassy's inflated data would more accurately represent a person's daily experience given the prevalence of both features.)

PM2.5 sensor locations in western and eastern Beijing

The Data
The numbers compared here cover February 2 to 4 for a total of 45 measurements. BJMEMC currently provides access to only the last 24 hours of data. Note that the embassy did not have data for 21:00 and 23:00 on February 2, so those times were omitted.

BJMEMC

U.S. Embassy

Difference

Average PM2.5 (μ/m3)

33.51

49.07

31.70%

Average Hourly Change

12.80

10.39

18.83%

AQI (U.S.) Equivalent

95

123

1 category

Beijing PM2.5 levels, Feb 2 - 4, 2012

BJMEMC's measurements were at times higher than those of the embassy, but on average the embassy recorded poorer air quality. The different was enough to bump the embassy's average into a more serious AQI category. Also, the BJMEMC data's hourly fluctuation was higher. On two occasions the BJMEMC readings dropped to 3 μ/m3 and remained there for two hours. This could be an error, especially at 18:00 February 3, when the reading jumps back to 111 μ/m3 an hour later.

Impressions
Overall, the results are consistent, but different enough to rank as "Moderate" at BJMEMC's sensor and "Unhealthy for Sensitive Groups" at the embassy. The important difference, though, is not between the BJMEMC and embassy data, but between BJMEMC's own PM10 and PM2.5 measurements. Up to now, official assessments of the air pollution were based on PM10 pollution, which does not have the same health implications as the finer PM2.5 pollution and was often measured in lower concentrations. For example, BJMEMC's assessment of the last 24 hours, based on PM10 at a nearby location, is "Excellent", which is not consistent with the visible pollution outside. With these new measurements, an informed public can now make health-conscious decisions.

But what about an uninformed public? Unfortunately, the web page does not translate its measurements into an AQI category and qualitative description (or "API" in the Chinese standard). Further increasing the potential for misinterpretation, the measurements are given in milligrams instead of the usual micrograms. And finally, the descriptions of the pollution levels don't seem consistent with the given health effects. For example, the description:

is matched with "Moderately polluted", a term that itself makes no implications about health.

The Website
Less satisfying than the data is the platform through which they are distributed. Unlike the embassy feed, which can take advantage of all of the distribution methods offered by Twitter, the BJMEMC data are trapped on a web page that is not even directly accessible from a URL. Instead, users must click a PM2.5 tab that loads the data in an iframe element using Javascript. The target page in that iframe cannot be accessed directly due to some validation techniques with Javascript and some hidden form fields. When we consider that the page only shows 24 hours at a time and does not provide an archive, it will be very difficult to extract these measurements in bulk for research purposes. Furthermore, the measurements are not updated consistently; the 21:00 measurement was the last added on February 3.

This was added to prevent users from selecting or copying the text on the page. Fortunately, it seems the site was developed on and for Microsoft applications and the controls do not work in Firefox, leaving users of that browser free to copy the text. These methods are supposed to be deprecated in WebKit as well, but I could not select any text in Chrome.

Regardless, I'll be exploring ways to capture the data in a more usable format.

Conclusion
The data quality is not perfect, but neither is the embassy's. The data access as it is currently provided can hardly be described as open, but it is definitely ajar, allowing citizens to peek in and maybe get a foot in the door to negotiate on some of the government's other data. This is a big step for such a secretive, paranoid authority, and the government deserves a pat on the back for it. Still, Beijingers need to push for:

Setting up more sensors in representative locations

Improving the data distribution platform and reducing the delay in publication

Expanding this practice into other datasets, especially in public health

Once BJMEMC gets a sensor near the embassy, I'd like to see another comparison.